One conventional technology uses CCD (charge coupled device) sensors, but it is apparent that a CMOS technology could also be used, this having the advantage of allowing both the actual image sensor, that is to say a matrix of photosensitive elements, and complex electronic image processing circuits to be produced on the same silicon integrated circuit chip. This CMOS technology also has the advantage of consuming less energy. However, it has the drawback of not allowing easy separation between acquisition of an image and reading of this image.
Typically, it would be desirable overall to acquire the entire image over an exposure time which not only is the same for all the picture elements (or pixels) but which starts at the same instant for all the elements and terminates at the same instant, before passing to an operation to read the electronic charges generated at each element by this exposure.
This is possible only if, at each pixel there is a means for storing the charges accumulated at this pixel, means for limiting the exposure time, that is to say the time to acquire the charges for all the pixels of the matrix, and there is a means for subsequently transferring, pixel by pixel, the charges stored in the pixel to the outputs of the matrix. These components were available in CCD technology and comprised the same number of storage regions as photodetection regions, these being located in a matrix contiguous with the photodetection region (frame transfer organization) or else located within each pixel (interline organization). However, in CMOS technology it is necessary to use several transistors in each pixel. These transistors, their connections and the row and column conductors allowing them to operate reduce the available space for producing an actual photosensitive region. As a result, there is reduced sensitivity for a given pixel size. Alternatively, at a given sensitivity the result is an increased pixel size and therefore an increase in the size of the chip and of the associated optical objectives, which is expensive, or else the result is a reduction in the number of pixels and therefore in the resolution of the sensor if the size of the chip is maintained.